Oscillator tracking system



u N 7 nu U RL 6 u.: a of. Oam MP o 8 o E C 2, w m 1c 2 w p 2W W ,5 ww u E tf n 6 Il x m 4 E M WM m m E 00 T u o G N H 1 MA w; M A s 4 M v s d H L G w so 2 s L N E m 5, ww h z M m c m w Mm H Tow C T u N M R J O w B u 1 L l G n F C s O u n M 9 m 1 4 m 1. w .w n J Patented July 14, 1942 OSCILLATOB TRACKING SYSTEM George B. McClellan, Chicago, Ill., assignor to Johnson Laboratories, Inc., Chicago, lll., a corporation of Illinois Application July 5, 1941, Serial No. 401,111

(Cl. Z50-40) Claims.

This invention relates to radio frequency systems and more particularly to systems, such as for example superheterodyne receivers, in which an oscillation generator is employed to transform a signal frequency into an intermediate frequency. In such systems it is desirable, and the universal practice, to provide a single control by which the frequencies of the signal frequency circuit or circuits, and of the oscillation generator, are simultaneously varied to tune the system over a range of frequencies. This imposes the condition that the circuit arrangements and the tuning means must be such that the signal frequency circuits and the oscillation generator will remain in continuous alignment at a substantially constant frequency diierence throughout the tuning range. The present invention is addressed particularly to the problem of securing this alignment or tracking in such systems. l

Tuning in systems of the class described is accomplished either by variation of the circuit capacitances using variable condensers, or by variation of the circuit inductances using variable inductors. The most convenient form of variable inductor is one in which inductance variation is secured by motion of a ferromagnetic core element relatively to an inductive winding, this method of frequency variation being called permeability tuning. The present invention relates particularly to systems in which this method of tuning is employed.

Various methods of securing alignment or tracking in uni-control signal frequency and oscillator systems have been proposed and are in general use. -Some of these methods are applicable to both variable-capacitance or variableinductance systems, while other methods are particularly adapted to one or the other system of tuning. The methods so far proposed for use in permeability tuned systems, while reasonably effective, nevertheless involve either additional coils or windings or special constructions of one or Vmore of the windings or cores, and therefore involve additional expense and complication.

In permeability-tuned systems as commonly employed, the ferromagnetic core is usually cylindrical and is arranged to be moved axially relatively to a winding. The winding is associated with one or more capacitors to form a resonant circuit and the resonant frequency of this circuit, for any position of the core relatively to the coil, depends upon the construction of the coil. If the coil is a simple uniformly wound solenoid, the tuning characteristic, that is, the

curve between core position and circuit frequency, will have a certain well-known shape. Such a coil is usually employed in the signal frequency portions of systems to which the present invention is addressed. Other coil-constructions will, in general, produce different tuning vcharacteristics.

If two resonant circuits are to be maintained at a constant frequency difference as they arev simultaneously tuned by a single control, the tuning ratios, that is, the ratios of maximum to minimum frequency, will be different, the circuit which is tuned to the upper frequencies having the lower tuning ratio. If the circuits are tuned by inductance variation, it is therefore necessary that the circuit which is to be tuned to the upper frequencies shall have a lower total inductance change than the circuit which is to be tuned to the lower frequencies. This result may be secured either by employing identical cores and constructing the winding in one circuit in such a way that the core will have a lesser effect upon it, or by employing different cores such that one has la lesser inductance changing effect than the other, or by employing both expedients in combination. Whichever method of adjusting the total frequency ranges is employed, there remains the problem of so constructing one or both of the windings and/or cores that the required frequency difference will be maintained throughout the tuning range.

In order to maintain a constant frequency difference between uni-control .permeability-tuned signal frequency and oscillator circuits throughout the tuning range it has been proposed to employ cores of different and complicated shapes i in the two circuits, or to employ variable magnetic densities in one or both of the cores, or to employ both expedients in combination. All of these methods, however, prove to be complicated and expensive, and they require costly laboratory development for each new design, the progress of the art being such as to require frequent changes.

It is a principal object of the present invention to provide a construction for the variable inductor for use in a circuit that is to bemaintained at a constant frequency difference above a second circuit, both circuits being tuned by the same control, which willbe simple and inexpensive and of readily determinable designconstants.

It is a further object of the invention to provide an arrangement for the described purpose in which those components which principally determine the frequency ranges and the tuning characteristics of the'signal and oscillator circuits will be simple in construction and therefore low in cost and capable of relatively inexpensive laboratory determination as to their required constants and construction.

It is an additional object of the invention, in

order further to insure low development and production costs, to provide an arrangement for the described purpose in which a 'minimum number of components are employed and in which the particular expedient by which alignment at a constant frequency difference is achieved, is

applied to only one element of the structure.

These and other objects and advantages of the Y .invention I achieve by the particular arrangements to be described in what is to follow.

In accordance with the invention, I secure alignment between signal-frequency and oscillator circuits at a substantially constant frequency difference by employing a combination of self and mutual inductances as the inductive portion of the oscillator circuit, and by simultaneously but differently varying these self and mutual inductances through the use of a rela- -tively movable ferromagnetic core ganged for uni-control operation with a similar core which tunes the preselector circuit.

Fig. 1 is a schematic diagram of a preselector and oscillator system embodying the invention;

Fig. 2 is a diagrammatic sectional drawingof of the oscillator windings, and the frequency difference initially established by adjustment of capacitor 5 is maintained substantially constant during motion of the core by virtue of the variation in mutual inductances produced by the particular construction of windings 2 andi in relation towinding I.

Windings I and 2 are wound in the same direction and therefore in aiding relation, whereas winding 8 is preferably wound in opposed relation vrelative to windings I and 2. This results ina slower rate of 'change of inductance as the core 4 first enters the windings I, 3' at the high-frequency end of the tuning range, and a relatively more rapid rate vof change of inductance as the core [completes its motion into the windings I, 2 at the low-frequency end of the tuning range. Thus, when the windings I, 2, 3 are properly constructed and the core 4 is properly chosen,

wayof illustrative example is of the conventional Colpitts type but that it employs a variable inductor having plural serially connected windings I, 2 and 3 andrelatively movable ferromagnetic core 4. 'I'he windings I, 2, 3 are so arranged that the effective inductance of the variable inductor, for any position of the ferromagnetic core, is determined not only by the individual inductances of the serially connected windings but also by the mutual inductances between them, which also vary in a predetermined and desired manner as the core 4 moves relatively to the windings In addition to the variable inductor just described, the oscillator circuit comprises serially connected capacitors 5, 6 having a grounded Junction, and vacuum tube 1 with the usual grid capacitor 8 and grid resistor 8,' all connected as shown.

Fig. 1 also shows a conventional permeabilitytuned preselector circuit supplying signal voltage to the mixer grid of vacuumtube 1. The tunable signal frequency circuit'comprlses inductance III and capacitors II and I2 serially connected as shown, the circuit being tuned to the desired signal by motion vof ferromagnetic core I3 which is ganged with core-4 for motion in unison by a uni-control indicated by the dash lines and nu` meral I4. `Capacitor II may be fixed in value and capacitor I2 may be adjustable for alignment purposes.

The reduced tuning range required in the oscillator circuit as compared with the preselector circuit is secured by the use of a slightly shorter core in combination with an increased diameter the differential action just described results in substantially perfect tracking between the oscil- .lator circuit and a preselector circuit employing a simple solenoidal winding.

In Fig. 3 of the drawing there is shown-an alternative arrangement of the variable ferroinductor of the oscillator in which only two windings, I1,A I8, are employed, winding I1 being identical with winding 21 of the antenna coil. The reduced frequency range required in the oscillator circuit is secured by the use of a lowdensity low-permeability core, and the frequency difference initially established by adjustment of capacitor 20 is maintained substantially constant during motion of the core by virtue of the variation in the mutual inductance produced by the particularA construction of the winding I8 in relation to winding I1. In the arrangement according to Fig. 3, winding I8 is wound in oppositionto winding I1 and has its turns variably spaced, as for example in the illustrative example to be described later. When the variable spacing of the turns of winding I8 is properly chosen, and when core I9 has the correct effective ypermeability, substantially perfect alignment can be se-A cured, maintaining a fixed frequency difference between the oscillation generator and the signalfrequency preselector circuit.

Still referring to Fig. 3, it will be apparent that as core I9 enters windings I1, I8, due to the opposed relation of these windings andas a result of the fact that the mutual inductance between them is increased by the presence of the core, the rate of increase of inductance will not be so great as would be the case if winding I8 were not present. However, winding I8 has its turns so spaced that its effect diminishes as the core I9 is advanced further into the windings so that at the lower frequency end of the tuning range the rate of change of inductance is accelerated.

I shall now give specific structural data for illustrative embodiments of the invention which are simple and inexpensive and have been found to give satisfactory results, it being understood that more elaborate and expensive constructions employing the principles of the invention may be used to achieve more precise alignment at con-A stant frequency difference, and that I do not Wish to be in any way limited to the particular constructions described.

'I'he windings and cores now to be described are suitable for the broadcast frequency range and for an intermediate frequency of l4:65 kilocycles.

Winding I 0 of Fig. 1 may suitably be a progressive universal winding of 1-44 single-silkenamel Litz wire, wound at 282 turnsper inch on a tube of .221-inch outside diameter and .205- inch inside diameter to a length of 11,. inches, and will have an inductance'in air of approximately 135 microhenrys. This winding may also be used in the preselector employed with the modified oscillator circuit of Fig. 3, and as the inner winding I1 of the oscillator coil system of Fig. 3.

The ferromagnetic cores l ,I3, I! and 30 may suitably have al cylindrical shape with a diameter of .200-inch, and should be constructed with ferromagnetic material that has been sifted through a standard 400 mesh screen. The particles should be coated with a standard insulating varnish, and the cores should preferably be molded with a powdered Bakelite resin binder. Thus core 4 (Figs. 1 and 2) and core I3 (Fig. l) may be constructed from a hydrogen-reduced tin-iron alloy containing about 6% by weight of tin. 'I'he particles should be carefully insulated and molded with 3% by weight of binder. Core 4 should have a length of 11/4 inches and a weight of 3.83 grams, and will have an effective permeability of 10.05. The same type of core may also be used as core 30 in the preselector system employed with the modified oscillator arrangement of Fig. 3. Core I 3 (Fig. l) should have a length of 1%-inches and a weight of 4.3 grams, and will have an effective permeability of 11.35. Core I9 (Figs, 3 and 4) should be made entirely of hydrogen iron using of binder, and should be 11/4 inches long, with a weight of 3.07 grams and an effective permeability of 5.8.

Windings I, 2 and 3 (Figs. 1 and 2) are suitably single layer solenoids of No. 38 plain enamel wire. Winding I may be wound at 200 turns per inch on a tube' of .S16-inch outside diameter to a length of 11% inches and will have an inductance in air of approximately 124 microhenrys. windings 2 and 3 may be wound on a tube of A23-inch' outside diameter slipped over Winding I. Winding 2 should start 11g-inch from the end of winding I and should have 15 turns wound at 60 turns per inch. Winding 3 should be wound in the opposite direction from windings I and 2 and should start 1/ inch from the end of winding 2. It should have 24 turns, the first 7 turns being spaced and the last 17 turns being wound close to give a total length of 1/z-inch for the 24 turns. Winding 2 will have an inductance of 4.8 microhenrys and winding 3 an inductance of 9.6 microhenrys. 'I'he mutual inductance between windings I and 2 is aiding and is 7.8 microhenrys, the mutual inductance between windings I and 3 is bucking and is 6.9 microhenrys,l

the mutual inductance between windings 2 and l is likewise bucking and is 2.15 microhenrys,

2 and 3 is also bucking and is 1.8 microhenrys. The connections of the windings to the grid and plate of vacuum tube 'I should be as shown in Fig. 2.

Winding I8 (Figs. 3 and 4) may suitably be a single layer solenoid of No. 38 plain enamel wire wound in a direction opposite winding II on a tube of .S27-inch outside diameter. Toachieve satisfactory alignment, winding I8 may have three sections, the first section starting 13g-inch from the end of winding I1 and having 20 turns close wound, the second section having 14 turns wound at 40 turns per inch and the third section having 82 turns close wound, the total winding length being 2%2-inch. 'I'his winding will have a self-inductance in air of approximately 'and the mutual inductance between windings I,

39 microhenrys. The mutual inductance be'- tween windings I'I and I8 is bucking and is 44.8 microhenrys. 'I'he connections from windings I'I and IB to the grid and plate of vacuum tube 22 should be as shown in Fig. 4.

Having thus described my invention what I claim is:

1. A tuning system including two circuits each having a coil and a relatively movable ferromagnetic core to tune said circuits over upper and lower frequency ranges respectively. and means for regulating the inductance variations produced by the relative movement of said coils and said cores to maintain said circuits at a substantially constant frequency difference, including a winding arranged in opposing series relation with the coil of the circuit operating over the upper frequency range.

2. A tuning system including two circuits each having a coil and a relatively movable ferromagnetic core to tune said circuits over upper and lower frequency ranges respectively, and means for regulating the inductance variations produced by the relative movement of said coils and said cores to maintain said circuits at a substantially constant frequency difference, including a winding disposed around and arranged in opposing series relation with the coil of the circuit operating over the upper frequency range.

3. A tuning sys-tem including two circuits each having a coil and a relatively movable ferromagnetic core to tune said circuits over upper and lower frequency ranges respectively, and a winding disposed around and arranged in opposing series relation with the coil of the circuit operating over the upper frequency range, said winding having a greater number of turns per inch at the end where the core enters than at the opposite end so as to produce less inductance variation with core movement in the last mentioned circuit at the high frequency end of the tuning range and a -greater inductance variation at the low frequency end of said range. v

4. A tuning system including two circuits each having a coil and a relatively movable ferromagnetic core to tune said circuits over upper and lower frequency ranges respectively, and means for regulating the inductance variations produced by the relative movement of said coils and said cores to maintain said circuits at a substantially constant frequency diiference, including a first winding arranged in series opposing relation with the coil of the circuit operating over the upper frequency range, and a second Winding arranged in aiding series relation with said same coil.

5. A tuning system including two circuits each having a coil and a relatively movable ferromagnetic core to tune said circuits over upper and lower frequency ranges respectively, a first winding arranged in opposing series relation with the coil of the circuit operating over the upper frequency range, and a second winding arranged in aiding series relation with said same coil, said first winding being disposed around said coil near the end Where the core enters and said second winding being disposed around said same coil near the opposite end thereof, so as to produce less inductance variation with core movement in the last mentioned circuit at the high frequency end of the tuning range and a greater inductance variation at the low frequency end of said range.

6. A tuning system including two circuits each having a coil and a relatively movable ferromagnetic core to tune said. circuits over upper and lower frequency ranges respectively, a first winding arranged in opposing series relation with the coil of the circuit operating over the upper frequency range, and a second winding arranged in aiding series relation with said same coil, said first winding being disposed around said coil near the end where the core enters and said second winding being disposed around said same coil near the opposite end thereof. at least one of said windings having more turns per inch in the portion nearest to its adjacent coil end than in the portion towards lthe middle of said coil so as to produce less inductance variation with core movement in the last mentioned circuit at the high frequency end of the tuning range and a greater inductance variation at the low frequency end of said range.

7. A superheterodyne receiver including a signal frequency circuit and an oscillator tank circuit, each having a coil and a relatively movable ferromagnetic core to tune said circuits over a range of signal frequencies and a corresponding range of oscillator frequencies respectively, a winding disposed around and arranged in opposing series relation with the coil of said oscillator tank circuit to so control theP inductance variation produced by the relative movement of coil and core in said oscillator tank circuit as to maintain a constant frequency difference between said circuits.

8. A superheterodyne receiver including a signal frequency circuit and an oscillator tank circuit, each having a coil and a relatively movable' ferromagnetic core to tune said circuits over a range of signal frequencies and a corresponding range of oscillator frequencies respectively, a winding disposed around and arranged in opposing series relation with the coil of said oscillator tank circuit, said winding having more turns per inch at the end where said core enters than at its central portion, to so control the inductance variation produced by the relative movement of the coil and core in said oscillator tank circuit as to maintain a constant frequency difference between said circuits.

9. A superheterodyne radio receiver including a signal frequency circuit and an oscillator tank circuit, each having a coil and a relatively movable ferromagnetic core to tune said circuits respectively over a range of signal frequencies-and a corresponding range of oscillator frequencies. a iirst winding arranged in opposing series relation with the coil of said oscillator tank circuit, and a second winding arranged in aiding series relation with said same coil, said first winding being disposed around said coil near the end where the core enters and said second winding being disposed around said coil a distance away from said first winding and near the opposite end ofsaid coil, so as to maintain a substantially contant frequency difference between said circui 10. A superheterodyne radio receiver including a signal frequency circuit and an oscillator tank circuit, each having a coil and aj relatively movable ferromagnetic core to tune said circuits respectively over a range of signal frequencies and a corresponding range of oscillator frequencies, a nrst winding arranged in opposing series relation with the coil' of said oscillator tank circuit, and

a second winding arranged in aiding series relation with said same coil, said first winding being disposed around said coil near the end where the core enters and said second winding being disposed around said coil a distance away from said first winding and near the opposite end of said coil, at least one of said windings having a varied winding pitch. so as to maintain a substantially constant frequency difference between said circuits.

GEORGE B. McCLELLAN. 

